linux-kernel - Re: [RFC] sched/dl: Fix preemption checks

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Date:	Wed, 22 Oct 2014 10:43:41 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Kirill Tkhai <ktkhai@...allels.com>,
	Juri Lelli <juri.lelli@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [RFC] sched/dl: Fix preemption checks

Hi Kirill,

On 21/10/14 17:35, Kirill Tkhai wrote:
> 
> 1)switched_to_dl() check is wrong. We reschedule only
> if rq->curr is deadline task, and we do not reschedule
> if it's a lower priority task. But we must always
> preempt a task of other classes.
> 
> 2)dl_task_timer():
> Policy does not change in case of priority inheritance.
> rt_mutex_setprio() changes prio, while policy remains old.
> 
> So we lose some balancing logic in dl_task_timer() and
> switched_to_dl() when we check policy instead of priority.
> Boosted task may be rq->curr.
> 
> (I didn't change switched_from_dl() because no check is
> necessary there at all).
> 
> I've looked at this place(switched_to_dl) several times
> and even fixed this function, but found just now...
> I suppose some performance tests may work better after this.
> 
> Completely untested, just RFC.
> 
> Juri how about this?
> 

I'll have to give it a spin, but it looks good.

Thanks,

- Juri

> Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
> ---
>  kernel/sched/deadline.c |   10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 256e577..937a875 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -532,7 +532,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>  	dl_se->dl_yielded = 0;
>  	if (task_on_rq_queued(p)) {
>  		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
> -		if (task_has_dl_policy(rq->curr))
> +		if (dl_task(rq->curr))
>  			check_preempt_curr_dl(rq, p, 0);
>  		else
>  			resched_curr(rq);
> @@ -1607,8 +1607,12 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
>  			/* Only reschedule if pushing failed */
>  			check_resched = 0;
>  #endif /* CONFIG_SMP */
> -		if (check_resched && task_has_dl_policy(rq->curr))
> -			check_preempt_curr_dl(rq, p, 0);
> +		if (check_resched) {
> +			if (dl_task(rq->curr))
> +				check_preempt_curr_dl(rq, p, 0);
> +			else
> +				resched_curr(rq);
> +		}
>  	}
>  }
>  
> 
> 
> 
> 
> 

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